The present invention relates to a method of manufacturing a semiconductor device comprising trench isolation. The invention has particular applicability in manufacturing high density semiconductor devices with submicron design features and active islands isolated by shallow insulated trenches.
Current demands for high density and performance associated with ultra large scale integration (ULSI) require submicron features of significantly less than 0.25 microns, increased transistor and circuit speeds and improved reliability. Such demands for increased density, performance and reliability require formation of device features with high precision and uniformity.
Conventional semiconductor devices comprise a substrate and various electrically isolated regions, called active islands, in which individual circuit components are formed. The electrical isolation of these active islands is typically accomplished by thermal oxidation of the semiconductor substrate, typically monocrystalline silicon, bounding the active islands. This type of isolation has been referred to as local oxidation of silicon, or LOCOS.
In further reducing the size of semiconductor devices, semiconductor-on-insulator (SOI) wafers increasingly have been used in very-large scale integration (VLSI) or ULSI of semiconductor devices. An SOI wafer typically has a thin layer of silicon on top of a layer of an insulator material. In SOI technology, the semiconductor device is formed entirely in and on the thin layer of silicon, and is isolated from the lower portion of the wafer by the layer of insulator material. In an SOI integrated circuit, essentially complete device isolation may be achieved using conventional device processing methods by surrounding each device, including the bottom of the device, with an insulator. One advantage which SOI wafers have over bulk silicon wafers is that the area required for isolation between devices on an SOI wafer is less than the area typically required for isolation on a bulk silicon wafer.
A type of isolation structure is known as trench isolation, wherein shallow isolation trenches are etched in the substrate between the sites of semiconductor devices and an oxide liner is thermally grown on the trench walls. The trench is then refilled with an insulating material. The resulting structure is referred to as a shallow trench isolation (STI) structure. STI has been widely applied to VLSI and ULSI semiconductor devices, and has been applied recently to SOI integrated circuits for such devices.
Trench isolation has several limitations, which may be exacerbated in SOI devices. One problem is that sharp corners at the top of the trench can result in gate leakage currents. More specifically, such sharp corners may cause unwanted increases in the sub-threshold currents in the channel regions along the edge of the device areas when the FETs are switched on. The device threshold voltage can also be lowered. In order to avoid these problems, it has been found desirable to round the corners of such trenches to increase the radius of curvature and thereby decrease the electric field at the corners. This has been accomplished by, for example, oxidizing the entire inner surface of the newly formed trench, taking advantage of the fact that an exposed corner of a silicon layer oxidizes faster than a flat surface of the silicon layer, thus forming a rounded upper corner at the top of the trench.
However, with SOI devices, the corner rounding solution leads to a new problem. The problem in SOI devices arises as a result of the proximity of the dielectric insulation layer below the silicon active layer. In SOI devices, the shallow isolation trench is etched through the silicon layer to the insulation layer. When the exposed portion of the silicon on the sidewalls of the newly formed trench is oxidized during the process of rounding the corners, a wedge or xe2x80x9cbird""s beakxe2x80x9d of new oxide may form on the underside of the silicon active layer adjacent the isolation trench, between the silicon active layer and the underlying layer of insulating material of the SOI wafer. Thus, as the oxide grows on the sidewalls of the trench, it may grow laterally between the lower edge of the silicon active layer and the underlying oxide insulation layer. In essence, during the process of oxidation which is intended to round the upper corner of the silicon active layer, the lower corner of the silicon active layer is also rounded, forming the xe2x80x9cbird""s beakxe2x80x9d between the silicon active layer and the underlying oxide insulation layer. The problem results when growth of the xe2x80x9cbird""s beakxe2x80x9d creates defects in the silicon crystal structure and/or lifts the silicon layer, due to the pressure of the growing oxide. The defects in the crystal structure may change the electrical characteristics of the semiconductor. The lifting of the silicon layer distorts the surface of the semiconductor device from its desired planarity to an undesirable non-planar condition. As semiconductor device dimensions continue to become smaller, problems such as these both occur more easily and become less tolerable.
Thus, there exists a need for STI methodology applicable to SOI semiconductor devices wherein the problems resulting from sharp corners can be alleviated without creating the problems resulting from xe2x80x9cbird""s beakxe2x80x9d on the underside of the silicon active layer adjacent the isolation trench.
The present invention provides a method of minimizing formation of the xe2x80x9cbird""s beakxe2x80x9d under the silicon island while providing rounded upper corners on the silicon active layer in shallow trench isolation of SOI semiconductor devices.
In one embodiment, the present invention relates to a method of isolation of active islands on a silicon-on-insulator semiconductor device, including the steps of:
providing a silicon-on-insulator semiconductor wafer having a silicon active layer, a dielectric isolation layer and a silicon substrate, in which the silicon active layer is formed on the dielectric isolation layer and the dielectric isolation layer is formed on the silicon substrate;
etching the silicon active layer to form an isolation trench wherein an unetched silicon layer at bottom of the isolation trench remains;
oxidizing the layer of silicon at the bottom of the isolation trench to a degree sufficient to oxidize through the layer of silicon at the bottom to the dielectric isolation layer; and
filling the isolation trench with a trench isolation material to form a trench isolation structure.
In one embodiment, the step of oxidizing results in oxidation of the silicon on sidewalls of the isolation trench. In one embodiment, the trench oxide liner and the dielectric isolation layer form an integral isolation structure for and define an active island.
In another embodiment, the present invention relates to a method of isolation of active islands on a silicon-on-insulator semiconductor device, comprising the steps of:
providing a silicon-on-insulator semiconductor wafer having a silicon active layer, a dielectric isolation layer and a silicon substrate, in which the silicon active layer is formed on the dielectric isolation layer and the dielectric isolation layer is formed on the silicon substrate;
etching the silicon active layer to form an isolation trench wherein an unetched silicon layer at bottom of the isolation trench remains;
oxidizing exposed silicon on sidewalls of the isolation trench and in the unetched silicon layer at the bottom of the isolation trench so as to electrically isolate from one another portions of the silicon active layer separated by the isolation trench and to round corners on an upper portion of the silicon active layer; and
filling the isolation trench with a trench isolation material.
In another embodiment, the present invention relates to a silicon-on-insulator semiconductor device, comprising:
a silicon-on-insulator wafer having a silicon active layer, a dielectric isolation layer a silicon substrate, and at least one isolation trench defining an active island in the silicon active layer, in which the silicon active layer is formed on the dielectric isolation layer and the dielectric isolation layer is formed on the silicon substrate;
wherein the at least one isolation trench includes a trench bottom liner which is an oxide formed from an unetched silicon layer in a lower portion of the isolation trench.
In one embodiment, the isolation trench above the trench bottom liner is filled with a trench isolation material. In one embodiment, the dielectric isolation layer, the trench bottom liner and the trench isolation material each comprise silicon dioxide. In one embodiment, the trench bottom liner is in contact with the dielectric isolation layer. In one embodiment, the dielectric isolation layer, the trench bottom liner, the trench isolation material and a sidewall oxide liner each comprise silicon dioxide.
Thus, the present invention provides methods of STI applicable to SOI semiconductor devices which do not suffer from problems resulting from formation of a xe2x80x9cbird""s beakxe2x80x9d on the underside of the silicon active layer adjacent the isolation trench, while still allowing formation of isolation trenches which provide for complete electrical isolation of adjacent active islands in the SOI wafer.